1. Field of the Invention
The present invention relates to a technique for subjecting a target substrate to a plasma process, and specifically to a plasma processing technique of the RF (radio frequency) discharge type that applies an RF to an electrode to generate plasma. Particularly, the present invention relates to a plasma processing technique utilized in a semiconductor process for manufacturing semiconductor devices. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
In manufacturing semiconductor devices and FPDs, plasma is often used for processes, such as etching, deposition, oxidation, and sputtering, so that process gases can react well at a relatively low temperature. In general, plasma processing apparatuses are roughly classified, in terms of the plasma generation type, into those utilizing glow discharge or RF discharge, and those utilizing microwave.
In general, a plasma processing apparatus of the RF discharge type includes a process container or reaction chamber, and an upper electrode and a lower electrode disposed therein in parallel with each other. The lower electrode is configured to support a target substrate (semiconductor wafer, glass substrate, or the like) thereon. The upper electrode or lower electrode is supplied with RF voltage for plasma generation through a matching unit. Electrons are accelerated by an RF electric field formed by the RF voltage and collide with a process gas, thereby ionizing the gas and generating plasma.
In recent years, miniaturization proceeds in the design rules used for manufacturing processes, and thus plasma processes are required to generate higher density plasma at a lower pressure. Under the circumstances, there is a trend in plasma processing apparatuses of the RF discharge type described above, such that the RF applied is selected from a range covering far higher frequencies (for example, 50 MHz or more) than conventional values (typically, 27 MHz or less). However, if the frequency of the RF discharge is set higher, when the RF power is applied from an RF power supply through a feed rod to the electrode backside, it is transmitted through the electrode surface by means of the skin effect and flows from the edge portion to the central portion on the electrode main surface (the surface facing the plasma). If an RF current flows from the edge portion to the central portion on the flat electrode main surface, the electric field intensity at the central portion of the electrode main surface becomes higher than the electric field intensity at the edge portion, so the density of generated plasma becomes higher at the electrode central portion than at the electrode edge portion. Further, the resistivity of plasma becomes lower at the electrode central portion having a higher plasma density, and, also on the counter electrode, an electric current concentrates at the central portion, so the uniformity of the plasma density is further lowered.
In order to solve this problem, a design is known in which the main surface central portion of an RF electrode is formed of a high resistivity member (for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-323456). According to this technique, the high resistivity member is employed for the central portion of the main surface (plasma contact surface) of an electrode connected to an RF power supply. The high resistivity member consumes more RF power as Joule heat there, so that the electric field intensity on the electrode main surface is more reduced at the electrode central portion than at the electrode peripheral portion. As a consequence, the low uniformity described above in plasma density is remedied.
However, in plasma processing apparatuses of the RF discharge type described above, the high resistivity member employed for the main surface central portion of an RF electrode may consume too much RF power as Joule heat (energy loss).